This proposal represents a close collaborative effort between the Jardetzky and Longnecker laboratories focusing on understanding Epstein-Barr virus (EBV) entry into target cells and in particular defining how receptor binding triggers fusion mediated by EBV-encoded glycoproteins. EBV is a causative agent in endemic Burkitt's lymphoma, Hodgkin's lymphoma, and undifferentiated nasopharyngeal carcinoma (NPC). EBV is also recognized as an important pathogen in immunosuppressed individuals including HIV/AIDS patients causing a variety of proliferative disorders such as immunoblastic lymphomas, oral hairy leukoplakia, and other pathologies. Fusion of EBV with a cellular membrane minimally requires a complex of viral proteins that includes gB, gH/gL, and gp42 for B cells and gB and gH/gL for epithelial cells. In the previous funding period, we made substantial progress in understanding how EBV as well as other herpesviruses bind to and ultimately fuse with target cells. In the past funding period, we solved three key structures - gp42 alone, which allowed comparison to our previous structure of gp42 bound to the receptor HLA, gH/gL, and the post fusion form of gB. In addition, in the previous funding period, we began to identify functional domains of these key glycoproteins providing considerable momentum to our proposed studies in the new funding period. Overall, in our two aims, we plan to elucidate how receptor binding to EBV triggers changes in viral glycoprotein interactions that ultimately result in refolding of gB and fusion of the virion envelope with a cellular membrane. Clarifying the interactions between cellular receptors and viral glycoproteins, and the steps that lead from receptor binding to membrane fusion, is essential for understanding the tropisms behind EBV associated diseases.